Cathodic electrodeposition coating (cathodic electrodip) is a process in which water-thinable coating materials are applied to electrically conducting workpieces by the application of a direct current. The workpiece is immersed in a paint bath and the ionized paint is deposited on the workpiece. The workpiece is then removed from the bath and in a further step the paint is baked. Preferred workpieces here include automobile bodies or parts thereof.
Added to the water-thinable primer systems are crosslinking catalysts such as dibutyltin oxide, for example. It has been observed that, when these catalysts are used, there are instances of cratering in which dibutyltin compounds, as metabolites of the dibutyltin oxide, are causally or cocausally involved. These craters are detrimental to the quality of the paint, and afterwork is frequently required. Hence, on an automobile finishing line, the paint must be sanded, for example, leading to additional costs. As a result of the craters, moreover, the perceived quality of the paint is reduced and its quality standing in the market suffers. Moreover, when using dibutyltin oxide and dioctyltin oxide, instances of precipitation have been observed in the dip tank. In the worst case, these precipitations can result in the loss of the entire dipping primer in the cathodic electrodip tank.
In the cathodic electrodip tank, as a result of entrainment of phosphate from the pretreatment process, there may be increased phosphate levels. If the crosslinking reaction of the electrodeposition coating is catalyzed by tin compounds such as dibutyltin oxide, for example, there may easily be formation of tin phosphate salts, which, as a precipitate, can lead to the formation of slight dents or even distinct craters (circular wetting defect down to the substrate). Moreover, there is a reduction in the level of catalyst in the dip bath, since its instability means that the dibutyltin oxide may gradually break down and may be removed from the bath material as a result of the aforementioned formation of precipitates. As a result there may be deficient crosslinking and a decrease in corrosion resistance.
Dibutyltin oxide and dioctyltin oxide have the disadvantage, furthermore, that they are solid compounds, and must therefore be used in relatively large amounts, as a result of their insolubility in the organic film material, thereby making these catalysts less economic. Since the material in question is a solid, it is also possible, moreover, for defects and craters to occur as a result of inadequate dispersing.
The prior art has disclosed a series of crosslinking catalysts, also called curing catalysts, which can be used in electrodeposition primer compositions, but which do not satisfy the exacting requirements made of a curing catalyst in dipping primers. Thus, for example, European laid-open specification EP 0 264 834 A1 discloses catalytically active metal compounds on organic polymeric supports as additions for an electrodeposition primer composition. EP 0 859 017 A1 describes non-water-compatible catalysts which are applied to a water-dispersible inorganic support in order that said catalysts can be used in aqueous systems too. The U.S. laid-open specification US 2007/0045116 A1 discloses an electrodeposition primer composition featuring a resin phase and catalytically active nanoparticles which are in dispersion in an aqueous medium. This resin phase comprises a curing agent and a resin containing active hydrogen. German patent application DE 100 41 038 A1 discloses SnO2-coated SiO2 particles suitable for the catalysis.
None of these examples discloses uniform distribution of the catalysts or of the catalyst-containing particles in their corresponding dispersions and/or dipping primer compositions. Furthermore, US applications U.S. 2007/0149655 A1, WO 2007/025297 A3, US 2007/0045116 A1 and US 2007/0051634 A1 present unsupported catalysts. In other words, material which in principle is catalytic is used even for the particle core, despite the fact that only the outer layers are catalytically active.
Accordingly it is an object of the present invention to provide electrodeposition primer compositions which comprise support-bound curing catalysts with homogeneous distribution.
This object is achieved by the technical teaching of the independent claims. Advantageous developments are found in the dependent claims, the description, the examples, and the figure.